"Performance is known to decrease over repeated sprints, yet the relative shares of physical and technical components are not known. The recent validation of an instrumented sprint treadmill allowed us to measure vertical (Fv), horizontal (Fh) and total (Ftot) ground reaction forces applied at each step during sprinting on the treadmill. The ratio Fh/Ftot was used to describe the effectiveness of force application (similarly to what has been proposed in cycling mechanics) and compute an index of propulsive technique (IPT)."

This study quantified the effects of repeated sprints on force and power output capabilities, and on the effectiveness of force application onto the ground. Trained Ss (N = 12) including three sprinters performed blocks of five 6-second sprints with 24-second passive rest periods between them and two minutes between blocks, until failure or four blocks (20 sprints) completed, on an instrumented motorized treadmill. Vertical, horizontal, and total ground reaction forces and belt speed were sampled at 1000 Hz, averaged for each push-off, and used to compute propulsive power as the product of horizontal force and speed. Another performance parameter was computed as the distance covered over four seconds. The index of propulsive technique was computed as the slope of the linear relationship between the ratio horizontal/total force and speed from the first step to that at top speed.

Ss performed series of 17.2 ± 3.6 sprints. For the first two sprints, propulsive power and distance covered over four seconds were 17.8 ± 2.0 W/kg and 30.3 ± 2.5 m, respectively. Propulsive power, total force, distance covered over four seconds, and the index of propulsive technique all decreased significantly by 26.4 ± 8.9, 6.20 ± 5.79, 13.5 ± 8.01, and 21.1 ± 19.5, respectively. The change in total force was significantly lower than the index of propulsive technique, and total force and the index of propulsive technique were not correlated during the first two sprints, nor were their changes correlated with fatigue.

Implication. Physical capabilities and performance are significantly reduced with repeated-sprinting fatigue. The technical ability to apply force effectively also decreases with sprinting fatigue. Sprinting fatigue appears to affect more than the technical ability of force application and the physical capacity of force production.